PCB Tech - Use a short-circuit tracker to find PCB short-circuit faults

The following is an introduction to using a short-circuit tracker to find PCB short-circuit faults

The first situation:

If the online function test result of U1 is: pin 4 is not flipped and the test fails, the user should switch the pin status window to impedance display mode,and compare the impedance to ground between pin 4 of U1 and other output pins. If each output pin If the impedance is equal, the internal function of U1 is damaged, and U1 should be replaced.

Measure the impedance of the 4th pin of U1 to ground. If the logic state of this pin is exactly low,its impedance will not be very low (greater than 10 ohms),if the pin is at a low high level, the impedance will be greater than 1 kiloohm.

Use a three-meter to measure the resistance of the 4th pin of U1 to ground. If it is approximately zero,you can use a milliohm meter to find the real short-circuit point through the "two-point positioning method" pioneered by Qtech. The QT25 and QT50 short-pcb trackers in Qtech system products are ideal instruments for finding short-circuit faults on circuit boards.

The second case:

Operation steps of "two-point positioning method":

First put the measuring probe of the milliohmmeter on the solder joint of the measured foot near the root of the pin (as shown in the figure below), set the range of the milliohmmeter to 200 milliohms, and measure a resistance value; then place the probe On the attached copper wire (as shown in the figure below),which is 3-4 mm away from the solder joint of the tested foot, read a resistance value. If the previous resistance value is less than the latter resistance value, it means that the short-circuit point is on the internal drive circuit of the chip under test.

The chip should undoubtedly be replaced at this time. If the previous resistance is greater than the latter, it means that the short-circuit point is not inside the chip under test, but on other chips connected to the outside of the chip, or on the PCB attached copper wire between the two.

The short-circuit resistance measured by the milliohmmeter depends on the degree of the short-circuit, but as long as the two resistance values measured by the "two-point positioning method" are different, the real short-circuit point can be determined.

The third case:

The real PCB fault point is that the 5th pin of U3 is short-circuited to ground (input pin internal transistor breakdown), but when the function test of U1 is performed, the test fails because the 4th pin cannot be flipped. Switch the fixture window to the impedance display mode, compare the impedance of the 4th pin of U1 with other output pins, and find that the impedance of the 4th pin is significantly lower than the impedance of the other pins (approximately zero).At this time,the user is not sure that the real fault is U1,and should use QT25 or QT50 short-printed circuit tracker for further measurement.First, use the "two-point positioning method" to determine whether the short-circuit point is inside U1 or outside U1.After measurement, it is found that the short-circuit point is outside U1.At this time,the user can find out all the chips connected to the 4th pin of U1 through the circuit diagram of the board under test or use the line tracking function of Qtech. Then use a short-pcb tracker to measure the resistance to ground of the pins connected to the 4th pin of U1 on these chips, and find that the 5th pin of U3 has the lowest resistance to ground. At this time,the possibility of short circuit between U1 and U2 will be eliminated, and the problem will be concentrated on the fifth pin of U3. Use the "two-point positioning method" again to determine that the short-circuit point is inside the fifth pin of U3. At this time, the U3 chip should be replaced.

However, if the U3 is tested for more certainty at this time, the result of the test may still be passed.This is because although the chip is short-circuited to ground at pin 5, there is still a certain resistance value. As long as the resistance value is greater than the minimum drive resistance value of the tester and the logic function inside U3 is not damaged, then the chip The functional test will pass. For example, like the 7400 NAND gate, if one of its input pins is shorted to ground, its function test can still pass. In this case, users can only determine the point of failure by comparing with the results of learning a good board. However, for this example, as long as the internal short-circuit of pin 5 of U3 is detected, it can be determined that U3 is damaged. Because it is impossible for PCB circuit designers to short-circuit the output pin of the chip to ground.

The fourth situation:

The 3rd pin of U2 is not completely short-circuited to ground.Use a three-meter to measure the resistance of this pin to ground,which is about 10 ohms.

The user must remember: the resistance of the output and input pins of the normal chip to the ground will not be between 10-40 ohms.

At this time,the PCB test of U1 will fail (because the 4th pin of U1 cannot normally drive such a low-resistance input pin),the screen shows that the 4th pin of U1 is in a low-resistance state. Use a three-meter to test the resistance of this pin to ground is also about 10 ohms.In order to find the exact location of the incomplete short circuit, ordinary milliohmmeters are powerless, because the measured leg has a resistance of about 10 ohms to the ground at this time.In order to solve this problem,the QT50 short-pcb track is designed to adjust the measurement zero position, which can shield the fixed resistance of 10-20 ohms.The specific operation steps are:set the measurement zero to 10 ohms and the range to 200 milliohms,and then perform the measurement according to the steps in the third case above.